The present invention relates to a circuit board and a method of producing the circuit board, more precisely relates to a circuit board, in which a base member and a cable layer have different thermal expansion coefficients, and a method of producing the circuit board.
Some test substrates, which are used for testing circuit boards, on which semiconductor elements will be mounted, and semiconductor wafers, include core substrates composed of carbon fiber-reinforced plastic (CFRP). In comparison with conventional glass-epoxy core substrates, thermal expansion coefficients of the core substrates composed of carbon fiber-reinforced plastic are small, and thermal expansion coefficients of the circuit boards having such core substrates can be corresponded to those of semiconductor elements to be mounted on the circuit boards. Therefore, thermal stress generated between a semiconductor element and a circuit board can be effectively avoided.
The circuit board is formed by laminating cable layers on the both side faces of the core substrate, and plated through-hole (PTH) sections are formed in the core substrate so as to mutually electrically connect the cable layers on the both side faces thereof. The plated through-hole sections are formed by boring through-holes in a substrate and forming plated layers (electrically conductive parts) on inner faces of the through-holes.
In case of the base member having the electrically conductive core section composed of, for example, carbon fiber-reinforced plastic, if the plated through-hole sections are formed by merely boring the through-holes and plating the inner faces thereof, the plated through-hole sections and the core section are electrically shorted. Thus, the plated through-hole sections are formed in the core substrate having the electrically conductive core section by the steps of: forming pilot holes, whose diameters are greater than those of the plated through-hole sections to be formed, in the base member; filling the pilot holes with insulating resin; and forming the plated through-hole sections in the filled through-holes. With this method, the plated through-hole sections and the core section are not electrically shorted (see JP Kohyo Gazette No. 2004/064467, JP Patent Gazette No. 2006-222216).
However, if the pilot holes are drilled, burrs are formed on inner faces of the pilot holes and the plated through-hole sections and the core section will be electrically shorted. To solve this problem, the inner faces of the pilot holes are coated with insulating layers so as not to electrically short the plated through-holes and the core section (see JP Patent Gazette No. 2006-222216). However, it is difficult to perfectly coat the rough inner faces of the pilot holes.
The core substrate is formed by laminating cable layers on the both side faces of a core section. If the core section is composed of a material having a small thermal expansion coefficient, e.g., carbon fiber-reinforced plastic, great heat stresses work to boundary faces between the core section and the cable layers because the thermal expansion coefficients of the cable layers are much greater than that of the core section (e.g., 13-30 ppm/° C.). By the great heat stresses, the cable layers will be separated from the core section or cracks will be formed therebetween. To avoid the problem, bonding strength between the core section and the cable layers must be increased. The problem of the heat stresses between the core section and the cable layers will occur in not only substrates having electrically conductive core sections but also generic substrates.